In recent years, it was proved theoretically and experimentally that performance of optical semiconductor devices employed as light sources in optical communication systems and high speed semiconductor devices such as HEMTs in computer systems are significantly improved by employing quantum well structures or superlattice heterostructures. Reduction in thickness and more precise control of chemical composition of layers constituting those structures are important to further improve the devices performance. A promising method for crystal growth to fabricate heterojunction structures and superlattice structures is MOVPE (Metalorganic Vapor Phase Epitaxy). For instance in growth of InGaAs/InP employing this method, trimethylindium (TMI) and triethylgallium (TEG) are decomposed to produce In and Ga atoms, and AsH.sub.3 and PH.sub.3 are decomposed to produce As and P atoms, whereby an InGaAs layer and an InP layer are epitaxially grown on a substrate. In the MOVPE method of growth of InGaAs/InP, it is difficult to make a steep heterointerface between the InGaAs layer and the InP layer because of a difference in vapor pressure between As and P. Various research organizations have been improving the steepness of the heterointerface utilizing a method of interrupting the growth of the epitaxial layers at the heterointerface. The following (1)-(3) are typical methods for evaluating steepness of the heterointerface:
(1) PL (photoluminescence) measurement PA1 (2) Measurement of two-dimensional electron gas mobility PA1 (3) X-ray diffraction analysis PA1 (1) a difference in diffraction angles between a main peak of X-rays which indicates an average lattice constant of the MQW structure and a peak which indicates the lattice constant of the substrate on which the MQW is grown, and PA1 (2) an intensity of a satellite peak which reflects the MQW periodic structure,
Although it is possible to macroscopically estimate the thin-film multilayer structure using any of these methods, it is difficult to evaluate respective heterointerfaces of the multilayer structure individually. For instance in the X-ray diffraction analysis which is usually used for evaluating steepness of the heterointerface, in order to evaluate an InGaAs/InP multiquantum well (MQW) structure, a calculated diffraction pattern is fitted to a measured diffraction pattern with respect to:
thereby to compare the design structure and the grown structure, whereby compositions and thicknesses of the metamorphic layers formed at the heterointerfaces are estimated. However, because these compositions and thicknesses of the metamorphic layers estimated by this method are averages of the whole MQW structure, if the MQW structure has minute periodic fluctuations in the compositions and the thicknesses of the metamorphic layers at the respective interfaces, it is difficult to detect those minute fluctuations.